Intracellular calcium (Ca2+) is the central regulator of cardiac contractility. A majority of Ca2+ ions entering cardiac cells via L-type calcium channels induce calcium release from the sarcoplasmic reticulum (SR). The resultant increase in intracellular free calcium, [Ca2+]i, boosts the amount of Ca2+ binding to the thin-filament protein troponin C, ultimately resulting in contraction. During relaxation, Ca2+ is removed from the cytosol via one of four transporters: two extracellular transporters (the sodium-calcium exchanger, NCX, and plasma membrane Ca2+-ATPase, PMCA), and two intracellular transporters (SR Ca2+-ATPase and mitochondrial Ca2+ uniporter). Previous attempts to estimate the relative contributions to Ca2+ efflux have shown that the NCX is the dominant efflux mechanism.
In addition to the important role of Ca2+ in the function of viable myoctyes, alterations in Ca2+ handling, primarily via increases in [Ca2+]i, provide a major contribution to irreversible ischemic damage. Cell death can result from one of several mechanisms following elevations in [Ca2+]i. Examples of these mechanisms include protease activation, membrane rupture, cell contracture, and gap junction dysfunction. Protocols that selectively inhibit the NCX are useful for investigating the roles of the NCX and PMCA, and have been shown to have potential therapeutic effects during myocardial ischemic-reperfusion injuries. When the NCX works as a pathway for Ca2+ entry, as it does during ischemia/reperfusion injury, the NCX inhibitor is expected to guard against Ca2+ overloading. SEA0400, a selective and potent NCX inhibitor, has been shown to provide protection from cardiac ischemia/reperfusion injury and from myocardial stunning.
Despite the established importance of Ca2+ for myocardial viability, no in vivo imaging technique exists to assess the Ca2+ content or changes in Ca2+ handling post-MI in the heart. Molecular contrast agent manganese (Mn2+) has been used as a surrogate marker to assess intracellular Ca2+ movement in vivo indirectly. Mn2+ has an ionic radius and chemical properties comparable to Ca2+, and is known to shorten the longitudinal magnetization relaxation time, T1, observed during manganese-enhanced magnetic resonance imaging (MEMRI). Furthermore, Mn2+ enters viable myocardial tissue via the L-type voltage-gated Ca2+ channels. The methods described herein can be used as a diagnostic tool in the early detection of abnormal Ca2+ handling in the myocardium, and for monitoring disease progression.